1. Field of the Invention
The present invention relates generally to a fabrication process for magnetoresistive devices of the CPP type (or CPP structure) adapted to read the magnetic field intensity of magnetic recording media or the like as signals, and more particularly to a process for the formation of a spacer layer that is a part of an essential device component and has influences on device's performance. The magnetoresistive device of CPP structure, for instance, may be used with hard disk drive systems, MRAMs, and magnetic sensors.
2. Explanation of the Prior Art
In recent years, with an increase in the longitudinal recording density of magnetic disk systems, there have been growing demands for improvements in the performance of thin-film magnetic heads. For the thin-film magnetic head, a composite type thin-film magnetic head has been widely used, which has a structure wherein a reproducing head having a read-only magnetoresistive device (hereinafter often called the MR device for short) and a recording head having a write-only induction type magnetic device are stacked together.
The MR device, for instance, includes an AMR device making use of the anisotropic magnetoresistive effect, a GMR device harnessing the giant magnetoresistive effect, and a TMR device tapping the tunnel-type magnetoresistive effect.
The reproducing head must have some characteristics in general, and high sensitivity and high output in particular. For the reproducing head capable of meeting such demands, there has already been a GMR head mass produced that makes use of a spin valve type of GRM device.
Such a spin valve type GMR device typically comprises, as part of the device, a spacer layer, a first magnetic layer (the so-called free layer) formed on one surface of the spacer layer, a second magnetic layer (fixed magnetization layer) formed on another surface of the spacer layer, and a pinning layer (generally an antiferromagnetic layer) formed in contact with the fixed magnetization layer that faces away from the spacer layer.
The free layer operates such that the direction of magnetization changes in response to a signal magnetic filed coming from outside, and the fixed magnetization layer has the direction of magnetization fixed by an exchange coupling magnetic field from the pinning layer (antiferromagnetic layer). With such device structure, MR changes are achievable via a difference in the relative angle of spins in two such ferromagnetic layers.
The structure of the spacer layer sandwiched between the first magnetic layer (the so-called free layer) and the second magnetic layer (the fixed magnetization layer) could be an imperative site that determines whether MR characteristics are good or bad. Applicant has already filed JP(A)2008-91842, proposing a specific multilayer structure best suited for the spacer layer. More specifically, Applicant discloses that the spacer layer is built up of a triple-layer structure comprising a first nonmagnetic metal layer and a second nonmagnetic metal layer, each one formed of a nonmagnetic metal material, and a ZnO semiconductor layer interposed between the first and the second nonmagnetic metal layer.
With the characteristics of the magnetoresistive device in mind, it would go without saying that of importance is what material is selected from the first and the second nonmagnetic metal layer. Yet, to allow the MR characteristics to work well, it has now turned out according to Inventors' intensive studies that another vital point is the method of forming the ZnO semiconductor layer that becomes the intermediate layer of the spacer layer, i.e., how to form that ZnO semiconductor layer. In particular, Zn has a melting point of as low as 420° C., and is more evaporable at lower temperatures in reduced pressure. For this reason, care must be taken of handling operation indigenous to Zn to which no attention has been paid so far.
Among prior arts that would appear to be relevant to the present invention, there is the one set forth in JP(A)2001-203408. The publication discloses a technique for forming an Al film while a substrate is being, or has been, cooled down. The publication states that the obtained Al film naturally oxidizes into an Al2O3 film; however, nowhere is any specific oxidization condition referred to. That Al2O3 film is quite different from ZnO, to which the fabrication process of the invention is to be applied, in terms of compound. Furthermore, the Al2O3 film disclosed in JP(A)2001-203408 should preferably be in an amorphous film structure form, whereas the ZnO film, to which the present invention is to be applied, should preferably have a film structure of good crystallizability: there is a sheer difference in the film characteristics demanded for both.
The situations being like this, the present invention has been made for the purpose of, with how to form the ZnO film in the spacer layer in mind, providing a fabrication process for magnetoresistive devices, which can not only achieve high MR ratios but have an area resistivity (AR) best suited for device fabrication as well. In terms of ideal morphology, the ZnO film should excel in both flatness and crystallizability.